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13. Remote Procedure Call (5.3)

13. Remote Procedure Call (5.3). Outline Protocol Stack Presentation Formatting. RPC Timeline. Client. Server. Blocked. Request. Blocked. Computing. Reply. Blocked. Caller. Callee. (client). (server). Return. Return. Arguments. Arguments. value. value. Server. Client. stub.

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13. Remote Procedure Call (5.3)

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  1. 13. Remote Procedure Call (5.3) Outline Protocol Stack Presentation Formatting

  2. RPC Timeline Client Server Blocked Request Blocked Computing Reply Blocked

  3. Caller Callee (client) (server) Return Return Arguments Arguments value value Server Client stub stub Request Reply Request Reply RPC RPC protocol protocol RPC Components • Stubs • Protocol Stack • BLAST: fragments and reassembles large messages • CHAN: synchronizes request and reply messages • SELECT: dispatches request to the correct process

  4. Sender Receiver Fragment 1 Fragment 2 Fragment 3 Fragment 4 Fragment 5 Fragment 6 SRR Fragment 3 Fragment 5 SRR Bulk Transfer (BLAST) • Unlike IP, tries to recover from lost fragments • still no guarantee of message delivery • Strategy • selective retransmission (by SRR) • aka partial acknowledgements

  5. BLAST Details • Sender: • after sending all fragments, set timer DONE • if receive SRR, send missing fragments and reset DONE • if timer DONE expires, free fragments • Two possibilities

  6. BLAST Details (cont) • Receiver: • when first fragments arrives, set timer LAST_FRAG • when all fragments present, reassemble and pass up • four exceptional conditions: • if last fragment arrives but message not complete • send SRR and set timer RETRY • if timer LAST_FRAG expires • send SRR and set timer RETRY • if timer RETRY expires for first or second time • send SRR and set timer RETRY • if timer RETRY expires a third time • give up and free partial message

  7. BLAST Header Format • MID must protect against wrap around • TYPE = DATA or SRR • NumFrags indicates number of fragments • FragMask distinguishes among fragments • if Type=DATA, identifies this fragment • if Type=SRR, identifies missing fragments

  8. Client Server Client Server Request Request 1 ACK Reply 1 Reply Request 2 Reply 2 ACK … Request/Reply (CHAN) • Guarantees message delivery • Synchronizes client with server • Supports at-most-once semantics Simple case Implicit Acks

  9. CHAN Details • Lost message (request, reply, or ACK) • set RETRANSMIT timer • use message id (MID) field to distinguish • Slow (long running) server • client periodically sends “are you alive” probe, or • server periodically sends “I’m alive” notice • Want to support multiple outstanding calls • use channel id (CID) field to distinguish • Machines crash and reboot • use boot id (BID) field to distinguish

  10. CHAN Header Format 0 16 31 • Type = REQ, REP, ACK, PROBE • CID = Channel ID, 64K concurrent logical channels between any pair of hosts • MID identifies each request/reply pair • BID = Boot ID, incremented each time the machine reboots Type CID MID BID Length ProtNum Data

  11. Client Server Caller Callee xCall xCallDemux SELECT SELECT xCall xCallDemux CHAN CHAN xPush xDemux xPush xDemux Dispatcher (SELECT) • Dispatch to appropriate procedure • Synchronous counterpart to UDP • Managing Concurrency • Address Space for Procedures • flat: unique id for each possible procedure • hierarchical: program + procedure number

  12. Examples of Hierarchical Procedure Numbers • Program: file server, name server • Within file server: • 1: read, 2: write, 3: seek • Within name server: • 1: insert, 2: lookup

  13. SELECT CHAN BLAST IP ETH Simple RPC Stack

  14. SunRPC • Sun’s Network File System (NFS) • IP implements BLAST-equivalent • except no selective retransmit • SunRPC implements CHAN-equivalent • except not at-most-once • UDP + SunRPC implement SELECT-equivalent • UDP dispatches to program (ports bound to programs) • SunRPC dispatches to procedure within program

  15. SunRPC • NFS Program number 0x00100003 • Getattr: 1 • Setattr: 2 • Read: 6 • Write: 8 • Port Mapper: maps program numbers to port numbers • has a program number 0x00100000. • run at a well-know port number 111

  16. 0 31 0 31 XID XID MsgType = CALL MsgType = REPLY RPCVersion = 2 Status = ACCEPTED Data Program Version Procedure Credentials (variable) Verifier (variable) Data SunRPC Header Format • XID (transaction id) is similar to CHAN’s MID • Server does not remember last XID it serviced • Problem if client retransmits request while reply is in transit

  17. Application Application data data Presentation Presentation encoding decoding … Message Message Message Presentation Formatting (7.1) • Marshalling (encoding) application data into messages • Unmarshalling (decoding) messages into application data • Data types we consider • integers • floats • strings • arrays • structs • Types of data we do not consider • images • video • multimedia documents

  18. (2) (17) (34) (126) Big- endian 00000010 0 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 1 1 1 1 1 1 0 (126) (34) (17) (2) Little- endian 0 1 1 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 1 0 High Low address address Difficulties • Representation of base types • floating point: IEEE 754 versus non-standard • integer: big-endian versus little-endian (e.g., 34,677,374) • Compiler layout of structures

  19. Taxonomy • Data types • base types (e.g., ints, floats); must convert • flat types (e.g., structures, arrays); must pack • complex types (e.g., pointers); must linearize • Conversion Strategy • canonical intermediate form • receiver-makes-right (an N x N solution) Application data structure Marshaller

  20. Interface descriptor for Procedure P Call P P Arguments Specification Arguments Code Code Client Stub Server type = stub compiler stub len = 4 value = 417892 INT Marshalled Marshalled arguments arguments RPC RPC Message Taxonomy (cont) • Tagged versus untagged data • Stubs • compiled • interpreted

  21. eXternal Data Representation (XDR) • Defined by Sun for use with SunRPC • C type system (without function pointers) • Canonical intermediate form • Untagged (except array length) • Compiled stubs

  22. type length type length value type length value value Abstract Syntax Notation One (ASN-1) • An ISO standard • Essentially the C type system • Canonical intermediate form • Tagged • Compiled or interpreted stubs • BER: Basic Encoding Rules (tag, length, value)

  23. Network Data Representation (NDR) • IntegerRep • 0 = big-endian • 1 = little-endian • CharRep • 0 = ASCII • 1 = EBCDIC • FloatRep • 0 = IEEE 754 • 1 = VAX • 2 = Cray • 3 = IBM • Defined by DCE • Essentially the C type system • Receiver-makes-right (architecture tag) • Individual data items untagged • Compiled stubs from IDL • 4-byte architecture tag

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